Polytetrafluoroethylene molding powder and process of preparing the same



United States Patent 3,528,955 POLYTETRAFLUOROETHYLENE MOLDING POWDER AND PROCESS OF PREPARING THE SAME Edward H. Lippman, Morrisville, Pa., assignor, by mesne assignments, to Liquid Nitrogen Processing Corporafion, Malvern, Pa., a corporation of Pennsylvania No Drawing. Filed May 16, 1967, Ser. No. 638,727 Int. Cl. C08f 3/24, 47/02 U.S. Cl. 26092.1 4 Claims ABSTRACT OF THE DISCLOSURE A fibrous non-porous polytetrafiuoroethylene (PTFE) molding powder exhibiting an anisotropic expansion factor not greater than 1.13 is prepared by milling granular PTFE at a temperature of at least 200 F. in a circular horizontal air mill having a grinding chamber in the shape of a horizontal toroid, the chamber having tangential jet orifices spaced equidistantly around the periphery of the chamber and wiper jet orifices spaced equidistantly between said tangential jet orifices.

BACKGROUND OF THE INVENTION Ultrafine fibrous PTFE is well known commercially and has been widely used in the manufacture of thin sheeting of large surface area. Such a sheet molding material is described in U.S. Pat. 2,936,301 and comprises a powder of ultrafine fibrous PTFE having an anisotropic expansion factor of between 1.16 and 1.28. This powder is prepared in an air swept hammer mill by grinding granular PTFE particles at a temperature between 19 and 327 C. followed by classifying the milled particles into the desired size range at a temperature not exceeding 90 C. The powder thus produced, when compressed in sheeting molds, yields strong preforms having high flex deflection properties as evidenced by the ability of large, thin unsintered sheets to be bowed up to 18 inches without crackmg.

Although this ultrafine fibrous PTFE has proved satisfactory in the production of large sheeting, its use in the manufacture of other molded products, such as precision parts, has been limited. Because of the high anisotropic expansion factor of this fibrous material, a high degree of shrinkage results upon sintering which in turn, causes flaws in the molded product. Because of the shrinkage which results from using such materials, it is necessary to allow for dimensional change in constructing the molds or to use two sets of molds, which procedures, in addition to being inconvenient, do not consistently produce acceptable products.

SUMMARY OF THE INVENTION According to the present invention, it has now been found that ultrafine fibrous PTFE having a low anisotropic expansion factor can be prepared by grinding PTFE granules at elevated temperatures in a circular air mill. More particularly, the present invention provides a method of preparing ultrafine fibrous PTFE having an anisotropic expansion factor not greater than 1.13 which comprises subjecting PTFE granular particles to milling in a circular air mill at a temperature of at least 200 F., classifying the particles into the desired size range and collecting the milled particles. The milling is effected substantially by interparticle collision.

The advantages oifered by the present method are numerous. By utilizing a circular air mill, classifying of the milled particles into the desired size range can be carried out simultaneously with milling. Drying can also be effected simultaneously with milling so that the 3,528,955 Patented Sept. 15, 1970 granular feed may contain a substantial amount of water and need not be pre-dried as required in prior methods. Because fiber length increases with increasing temperature, the present method makes it possible to produce tailor-made molding powders for a wide variety of applications where a certain degree of fibrosity together with a low anisotropic expansion factor are essential.

The ultrafine fibrous powders obtained, besides having a low anisotropic expansion factor, have excellent handling characteristics. They can be readily loaded and levelled in molds and easily compressed at conventional molding pressures to yield strong preforms which resist cracking during handling and during transportation from the mold to the furnace as evidenced by their high unsintered flex deflection properties. Upon sintering, the preforms exhibit unusually low shrinkage and a surface roughness greatly superior to preforms fabricated from previously available ultrafine fibrous PTFE powders. Such results are quite unexpected since heretofore it was believed that milling temperature of 200 F. and above would produce fibrous material having a high anisotropic expansion i.e., exceeding 1.13, and also, that fibrous PTFE powders produced in a circular horizontal air mill would yield fragile preforms.

PREFERRED EMBODIMENTS OF THE INVENTION In carrying out the process of the present invention, a commercial grade of PTFE granular particles is fed into a circular air mill. Illustrative of the raw PT=FE granular powders which may be employed are those described in U.S. Pat. 2,393,967 obtained by contacting tetrafluoroethylene in the absence of organic additives with an agitated aqueous solution of an inorganic peroxide catalyst. The granular feed material may be substantially dry or may contain up to about 50% by weight water.

The term granular PTFE is used herein to refer to PTFE resin in the form of rough irregular particles of supercolloidal size having an average measured diameter (wet-sieve size) of 50 microns or above as determined in a conventional manner by actually sieving the particles. Granular powder is to be distinguished from fine powders obtained by coagulation of aqueous dispersions of colloidal PTFE which are not suitable for general molding applications and from ultrafine PTFE powder as used herein to denote the powders produced in accordance with the present invention by milling granular PTFE particles to obtain ultrafine particles having an average measured diameter (wet-sieve size) less than 50 microns.

The circular air mill used may be any of those commercially available, such, as the Micronizer, Micron Master" and Jet Pulverizer, provided it is equipped with means to substantially prevent the particles from colliding with the chamber wall during milling. This type of mill can be described as a hollow horizontal toroid and consists of a shallow circular grinding chamber in which the material introduced into the mill through a feed injector is acted upon by a number of gaseous fluid jets issuing through orifices spaced around the periphery of the chamber. The grinding chamber varies from 2 to 48 inches in diameter and from 1 to 2 /2 inches in axial height. The jet orifices, which may vary in diameter up to A inch, are drilled through the peripheral wall of the grinding chamber and are positioned tangentially so that the entering fluid jet will promote rotation of the feed in one direction. The tangential jets may range from 3 to 16 in number and are spaced equidistantly around the circumference of the chamber. For the present process, the mill is modified by the addition of wiper jets which are equally spaced between the tangential jets. The orifices for the wiper jets, which also may vary up to A inch in diameter, are drilled through the peripheral wall of the chamber but are angled so that the issuing fluid jet will be substantially parallel rather than tangential to the wall in order to keep the particles away from the wall during milling.

When a high energy fluid, such as air or an inert gas under pressure, is introduced into the chamber, the fluid pressure is converted into velocity head by expansion to substantially atmospheric pressure which causes a highspeed rotationof the feed material to be pulverized. Although the centrifugal force of rotation causes the PTFE material to concentrate at the periphery of the chamber, the wiper jets substantially prevent the particles from impinging on the chamber wall with the result that particle size reduction is effected substantially by interparticle collisio-n. The air or other fluid supplying the grinding energy is withdrawn at an inward point tending to cause the particle-laden gas to travel spirally. The smaller particles are carried out with the gas, and the coarser particles which are thrown to the periphery of the grinding chamber, are subjected to further reduction so that grinding and classifying to separate suitable fine particles of the desired size are carried out simultaneously in the grinding chamber of the mill. The reduced and classified particles issue through an outlet in the chamber which leads directly into a concentric centrifugal collector where the particles are separated from the grinding fluid and collected.

The temperature during milling should be at least 200 P. so that the ultrafine PTFE powder obtained will be composed essentially of fibrous particles or contain at least a substantial proportion of fibrous particles. In general, temperatures ranging between 200 F. and 800 F. are satisfactory for producing ultrafine fibrous material having a low anisotropic expansion factor. Above 800 F. there is a tendency for the particles to become tacky and agglomerate.

The feed size, feed rate, fluid pressure and fluid volume i.e., air pressure and air volume, employed are related to the size of the mill and may be adjusted to give the desired particle size. For molding applications leading to the preparation of small sized molded objects of the order of /2 to inches, an average particle size less than 50 microns (wet-sieve size) and not exceeding 5 microns (subsieve size) is generally preferred. In producing particles of this size using a four-inch mill, the feed size usually ranges between about 50 and 1500 microns, and preferably, the average measured particle diameter of the raw PTFE granular powder ranges between 100 and 1000 microns. The feed rate may range up to pounds per hour; the air pressure between 50 and 110 pounds per square inch gravity (p.s.i.g.); and the air pressure between 50 and 75 standard cubic feet per minute (s.c.f.m.).

For the purposes of the present specification, the terms used above and others as will be used hereinafter are defined as follows:

Subsieve size is a measure of surface area and corresponds to the theoretical diameter of the average particle on the assumption that the particles are non-porous spheres. The value obtained refers to the calculated specific surface particle diameter as determined by the air permeability method using, for example, the Subsieve Sizer catalog number 14-312 of the Fisher Scientific Company. According to the air permeability method, air is passed upward through a layer of particles. Particles with a greater surface area will provide a greater resistance to air. The resistance is measured and related to surface area which, in turn, is related to particle size.

Anisotropic expansion factor is a measure of the dimensional change obtained on sintering as follows: Four and one-tenth grams of powder are weighed into a half-inch square rectangular mold cavity and compressed between metal plugs. Pressure is built up to 4000 p.s.i. during one minute, held for an additional two minutes, and then released. The resulting roughly cubical preform is allowed to stand for 30 minutes in air at room temperature. The width, breadth and height of the preform are then measured (i.e., the X, Y and Z axes, respectively, where Z is the axis compressed during preforming). The measured preform is then baked 30 minutes at 716 F. to obtain a sintered piece, allowed to cool in air to room temperature, and rerneasured. The anisotropic expansion factor is the value of Z /Z divided by Percent shrinkage Original diameter Sintered diameter Original diameter Shrinkage is determined by molding a 2% inch-diameter disc from 28 grams of powder at 4000 p.s.i. and measuring the diameter of the disc after it has been sintered for 2 hours at 716 F. and cooled to room temperature at a rate of about 2 F./minute.

Tensile strength is a measure of the greatest longitudinal stress that can be applied before rupture and is reported in pounds per square inch (p.s.i.). In determining tensile strength, 2% inch-diameter discs molded at 4000 p.s.i. and sintered at 716 F. are machined to a thickness of 0.040 inch. Specimens are cut from the machined discs and tested on an Instron tester using an initial jaw opening of 0.875 inch and a jaw speed of 2 inches per minute.

Elongation is a measure of the total stretch in the direction of load expressed as a percentage of original length and is determined by measuring the elongation at break of specimens cut from machined discs, as prepared for tensile measurements, on an Instron tester using an initial jaw opening of 0.875 and a jaw speed of 2 inches per minute. The elongation i.e., length at break is then divided by the original length of the specimen.

Unsintered flex deflection is determined by molding A2" x 10 /2" x 10 /2" sheets at 2000 p.s.i.; supporting the sheet at two edges; applying a force to bend the sheet in the center; and measuring the height of the arc at the rupture of the sheet. The flex deflection is reported in inches.

Surface roughness is a measure of surface irregularities expressed in microinches as determined by means of a Brush Surfindicator at cut-off lengths of .003, .010 and .030 inch using 2%" diameter-28 gram discs molded at 2000 p.s.i. In this test a diamond stylus is passed over the surface of the sample, and the irregularities are amplified so that they can be readily detected on a meter attached to the instrument. The value is an average of microinch measurements over the surface of the specimen.

The following examples are given to illustrate the present invention more clearly, but are not intended to limit the scope thereof.

EXAMPLE 1 Three samples of granular PTFE powder having an average particle size of 500 microns and containing about 10% by weight water were separately milled in a fourinch circular air mill at a temperature of 300, 400 and 500 F respectively. The mill employed was a Micron Master Model No. 4 having 4 tangential jets and modified by the addition of 4 wiper jets. All samples were milled at a rate of 10 pounds per hour using a jet circle of 2 inches, a jet size of 0.083 inch, and 50 s.c.f.m. of air at 100 p.s.i.g.

EXAMPLE 2 Three samples of dry granular PTFE- powder having an average particle size of 500 microns were separately milled at a temperature of 300, 500 and 800 F., respectively, using the circular air mill of Example 1. All samples were milled at a rate of 6 pounds per hour using a 2% inch jet circle, 0.083 inch jet size and 50 s.c.f.m. of air at 100 p.s.i.g.

EXAMPLE 3 Five samples of granular PTFE powder having an average particle size of 650 microns were separately milled in the four-inch circular air mill used in Example 1 at a temperature of 200, 300, 400, 500 and 600 F., respectively. The granular feed material contained approximately 20% by weight water. All samples were milled at a feed rate of about 12.5 pounds per hour using a 2% inch jet circle, 0.083 inch jets and 50 s.c.f.m. of air at 100 p.s.i.g.

All of the powders produced in the above examples comprised an ultrafine fibrous, non-porous PTFE powder having a specific gravity of 2.16-2.17 and a theoretical average particle size (subsieve size) not greater than 5 microns with the average particle length ranging between 100 and 1000 microns and the average particle diameter ranging between 5 and 20 microns. The anisotropic expansion factor and other properties of the products obtained are set forth in the following table.

TABLE Milling Anisotro- Shrink- Tensile Example temp. pic expanage, perstrength Elongation No. (*F.) S1011 factor cent (p.s.i.) (percent) EXAMPLE 4 Several samples of granular PTFE powder having an average particle size of 500 microns and containing about 30% by weight water were milled at temperatures ranging between about 200 and 400 F. according to the procedure described in Example 2 above. The anisotropic expansion factor and certain other properties were determined and an average of the results showed a subsieve particle size of 3.3; and anisotropic expansion of 1.11; a percentage shrinkage of 3.7; an unsintered flex deflection of 1.78; and a surface roughness of 50, 34, and 16 microinches at cut-off lengths of .030, .010 and .003 inch, respectively.

By way of comparison, these same properties were determined for a commercial ultrafine fibrous, non-porous PTFE powder produced in an air swept hammer mill. The results obtained showed a subsieve particle size of 4.4; an anisotropic expansion factor of 1.27; a percentage shrinkage of 7.2; an unsintered flex deflection of 1.63 and a surface roughness of 82, and 37 microinches at cutoff lengths of .030, .010 and .003 inch, respectively.

From a comparison of the properties of the two fibrous powders, it is readily apparent that the powder produced according to the present invention exhibits excellent preform strength, as apparent from its unsintered flex deflection properties which even exceeds the value obtained for the commercial fibrous powder. In addition, the ultrafine powder produced in accordance with the present method upon sintering, exhibits greatly improved shrinkage characteristics and excellent surface properties that are similar to those obtained with substantially non-fibrous powders composed of rounded PTFE particles where surface roughness value are 48, 36, and 28 microinches under the testing conditions specified above.

I claim:

1. A process for preparing an ultrafine fibrous polytetrafluoroethylene having an anisotropic expansion factor not greater than 1.13 which comprises (a) subjecting polytetrafiuoroethylene granular powder to milling in a circular air mill at a temperature of at least 200 F., said circular air mill having a grinding chamber in the shape of a horizontal toroid, tangential jet orifices spaced equidistantly around the periphery of said grinding chamber, and wiper jet orifices spaced equidistantly between said tangential jet orifices, said milling being done by introducing the polytetrafluoroethylene granular powder into said grinding chamber and introducing air under pressure through said tangential and wiper jet orifices whereby the granular powder is rotated at high speeds in one direction causing interparticle collision, (b) classifying the milled particles to separate therefrom particles having a subsieve size not exceeding 5 microns, and (c) collecting said subsieve sized particles.

2. A process of claim 1 wherein classifying of the milled particles is effected simultaneously with milling.

3. A process as in claim 1 wherein milling is carried out at temperature between 200 and 800 F.

4. A process as in claim 1 wherein said granular powder contains up to about 50% by weight of water.

References Cited UNITED STATES PATENTS 2,936,301 5/1960 Thomas 26092.l

JOSEPH L. SCHOFER, Primary Examiner J. A. DONAHUE, JR., Assistant Examiner 

